The invention relates to a method of writing a pattern on a substrate by means of an electron beam.
It is common practice in electron beam lithography to write patterns on wafers, masks, reticles and other substrates by fracturing the patterns, i.e. breaking down each pattern into sub-patterns and by exposing the sub-patterns in sequence to build up a composite image. The pattern is conventionally divided into a grid format of square main fields and each main field is further divided into a grid format of square sub-fields. Each subfield is written by beam deflection to scan the subfield area, usually by vectoring of the beam to individual pattern shapes within the area by the most economic sequence of travel and by boustrophedon scanning of each shape. At the conclusion of each sub-field scanning the beam is moved on to the next sub-field and the process is repeated. When the pattern features of all sub-fields have been written, the substrate itself is displaced to position a succeeding main field centrally of the beam axis, so that its sub-fields can in turn be written.
The main field sizes in such fractured patterns are typically in the order of 0.1 to 4.0 millimeters square and in excess of 1,000 sub-fields are generally present on each field. The sub-field size principally depends on the fineness of the pattern features; features with a 1.0 micron dimension may be written by, for example, a resolution represented by 0.1 micron steps in the beam deflection, resulting in a sub-field size of about 100 microns square. To minimize or avoid distortion of pattern features which cross sub-field and main field boundaries or misalignment of features interconnected at these boundaries, it is necessary to apply corrections to the beam deflection system on each transition between sub-fields and main fields. These corrections compensate for, for example, deflection distortion, magnetic effects and time-related drift effects.
The afore-described lithographic techniques are in general use in the industry and are summarised in, for example, Jones and Dix: Electron Beam Lithography in Telecommunications Device Fabrication, Part 1, British Telecom Technology Journal, Vol. 7, No. 1.
Whilst such techniques are successful in microlithography, difficulties arise if they are applied to nanolithography. Nanolithography patterns are typically characterised by extremely fine features grouped centrally and coarser features graduating out towards the edges of the pattern. Writing the pattern requires the sub-fields to be only a few microns square, which results in correspondingly small main fields and sub-fields. This in turn significantly increases scanning time for the entire pattern. In addition, alignment of features at sub-field and main field boundaries is more difficult, particularly at transitions of pattern fineness.